OF= 1
Let the side length of a square be x.
Because ∠ o = 45.
So OD=x
So OE=2x
Because of OE? +EF? =OF?
So (2x)? +x? = 1?
Solution: x=√5/5
So the side length of a square is √5/5.
Then subtract the area of triangle and square from the area of big fan.
Finally it should be equal to 1/8π-3/ 10.
(I wonder if there is any miscalculation)
(voiceover: ahem ... classmate, is "a difficult math problem in the senior high school entrance examination" exaggerated? -_-|||)